Cochliomyia hominivorax, known as the New World screwworm, is a parasitic fly species. Unlike most flies that feed on dead tissue, its larvae consume the living flesh of warm-blooded animals. This makes it a destructive pest, capable of harming livestock, wildlife, and occasionally humans.
What is Cochliomyia
Cochliomyia is a genus of flies known as blow flies. Cochliomyia hominivorax is distinct because its larvae are obligate parasites, meaning they must feed on living tissue to survive. Other species, like Cochliomyia macellaria (secondary screwworms), primarily feed on decaying or dead tissue.
The key difference between C. hominivorax and C. macellaria lies in their feeding habits and larval morphology. C. hominivorax larvae burrow into healthy flesh, possessing pigmented tracheal trunks and encircling bands of spines. C. macellaria larvae feed on necrotic tissue and have non-pigmented tracheal trunks and V-shaped spines.
Life Cycle of the Screwworm
The life cycle of Cochliomyia hominivorax begins when a reproductively mature female fly seeks out fresh wounds on warm-blooded animals. These wounds, which can result from injuries, dehorning, castration, or even insect bites, attract the female fly due to the odors produced by bacteria. She then lays a mass of 10 to 400 eggs on or near the edges of the wound.
Within 12 to 24 hours, the larvae, also known as screwworms, hatch from these eggs. Immediately upon hatching, they burrow into the host’s living tissue, feeding and enlarging the wound. They develop through three larval stages, during which they grow to lengths of 6.5 to 17 mm and become whitish to cream-colored.
After approximately 5 to 7 days of feeding on the host, the mature larvae drop from the wound to the ground. They then burrow into the soil to pupate, a stage that can last about 7 days but may extend to 54 days depending on temperature and humidity. The adult fly emerges from the pupal casing, completing the entire life cycle in about 20 to 21 days under optimal warm conditions.
Screwworm Infestations (Myiasis)
Myiasis is the infestation of living vertebrate tissue or fluids by fly larvae, and screwworm myiasis refers to the infestation caused by Cochliomyia hominivorax. The presence of screwworm larvae in a wound is often indicated by a foul-smelling discharge and a socket-like, circular appearance. The larvae burrow deeply into the tissue, making them difficult to see initially.
Infestations affect a wide range of warm-blooded animals, including livestock such as cattle, sheep, and goats, as well as pets and wildlife. Newborn animals are particularly vulnerable due to their umbilical cords, which can serve as entry points for the flies. Animals suffering from screwworm myiasis may exhibit irritation, discomfort, lethargy, and weight loss. If left untreated, the continuous feeding of the larvae can lead to extensive tissue destruction, secondary bacterial infections, and ultimately, death within 5 to 10 days.
Historically, screwworm infestations had a severe economic impact on agriculture across the Americas. Before successful eradication efforts, annual losses for the US livestock industry were estimated at approximately $540 million. While human cases are less common, they can occur in individuals who tend livestock or travelers to endemic areas, particularly those with open wounds. Symptoms in humans include unexplained, painful, and worsening skin lesions, bleeding, a foul odor, and the sensation of larval movement, with severe cases potentially being fatal.
Eradicating the Screwworm
The primary method for eradicating Cochliomyia hominivorax is the Sterile Insect Technique (SIT). This biological control method involves mass-rearing screwworms in specialized facilities. At the pupal stage, male flies are sterilized using gamma radiation, which damages their sperm, preventing viable offspring.
Billions of these sterile male flies are then released by aircraft into infested areas, vastly outnumbering the wild male population. When a sterile male mates with a wild female, the female lays unfertilized eggs that do not hatch, as female screwworms typically mate only once in their lifetime. This leads to a progressive reduction in the wild screwworm population and, eventually, its eradication.
SIT has eliminated C. hominivorax from significant regions. The United States was declared screwworm-free in 1966, followed by Mexico in 1991, and various Central American countries by 2000. A permanent barrier zone is maintained in eastern Panama to prevent reintroduction of the fly from endemic areas in South America. This ongoing program, supported by the Panama-United States Commission for the Eradication of the Screwworm (COPEG), involves the continuous release of sterile flies from a production facility in Pacora, Panama.